What's stopping you from buying new non-runflat tyres for your stock wheels and then transferring them to a set of GTC wheels when they're back in stock? Stock wheels will run a 225/40 & 255/35 R18 which will transfer fine to the slightly wider GTC's (unless you want 19inchers).
I'm not up to speed with runflat tech - it is only the tyres that dictate runflat rather than the wheels too correct?